1. Field of the Invention
This invention relates to a method of removal of acid components from a gas, and more particularly, a new and improved process for removal of carbon dioxide from gas mixtures.
2. Prior Art
In preparation of the gases for ammonia synthesis or of other products, it is necessary that a large quantity of carbon dioxide is removed from the initial gas mixture. In order to remove CO.sub.2 from such gases, a physical absorption process is now very widely used which is based on utilization of ethers of polyglycols. The utilization of such solvents allows the investment and operation cost to be considerably cut compared with the known industrial processes of gas purification with ethanol amines and hot potash solutions (Spravochnik Azotchkika, 2nd issue, Moscow: Khimiya, 1986).
It is known that when ethers of polyglycols are used as absorbents of acid components, these solvents absorb also moisture from the gases to be treated and from the air used to strip acid components from saturated absorbents. But if the solutions of ethers of polyglycols contain water, this considerably reduces their absorption capacity for carbon dioxide. According to the experimental data obtained by the inventors, the CO.sub.2 solubility in dimethyl ether of triethyleneglycol which contains 0 to 10 wt. % of water at the standard conditions (0.degree. C. and 1 atm. or 760 mm Hg) decreases by 18% (see Table 1). At the other conditions, the solubility decreases similarly.
The decrease of the absorption capacity of an absorbent causes however an increase of the power consumption for its circulation as well as the deterioration of the carbon dioxide removal from the gas and the impossibility of the permanent maintenance of a constant composition of the gas purified.
TABLE 1 ______________________________________ Carbon Dioxide Solubility (Nm.sup.3 /m.sup.3) in Dimethyl Ether of Triethyleneglycol at Different Water Contents (0.degree. C.) CO.sub.2 partial pressure H.sub.2 O content of absorbent, % wt. atm. 0 5 10 1 2 3 4 ______________________________________ 1 7.7 7.2 6.3 2 15.2 14.2 12.5 4 30.8 28.3 25.3 6 46.3 42.6 38.0 8 63.4 60.2 55.2 10 81.5 77.4 70.9 ______________________________________
The technological calculations show that for an effective industrial operation of absorbents based on ethers of polyglycols, it is necessary the water content of the absorbents to be maintained at the minimum attainable level, 2 to 2.5 wt. %. According to this requirement, it is necessary the water to be permanently extracted from the operating circulating solutions of absorbents. It is known that this purpose is now attained by the methods as follows. The most widespread method is that according to U.S. Pat. No. 2,649,166 when a certain par of the degassed solvent extracted from the desorber bottom is delivered to the dehydration evaporator. To facilitate the water evaporation process and to minimize the possible decomposition and evaporation of the solvent, a vacuum (P.sub.res =60 mm Hg) shall be maintained in the evaporation column at the evaporation temperature of 100.degree. C.
The shortcoming of this method is considerable power consumptions to make vacuum for evaporation of water (see Table 4).
In U.S. Pat. No. 3,837,143 another method is described for removal of acid components from gases with esters of polyglycols where the whole circulating solvent which contains up to 10 wt. % of water is delivered for dehydration into an evaporation zone where it boils for evaporation of the water which it contains. This evaporation is realized with help of the steam generated within the system. The quantity of the vapor extracted from the stripper shall be equal to that of the water removed plus the excess quantity of water up to 50 wt. %. The water necessary according to the balance is extracted from the system, and the excess condensed returns back into the stripper. The shortcoming of this method is the necessity of heating the whole quantity of circulating absorbent up to a high temperature (e.g. the boiling point of dimethyl ether of triethyleneglycol which contains 10% of water is equal to 120.degree. C.), which will cause an accelerated degradation of the organic solvent and, consequently, an increased consumption of reagents as well as additional heat losses while cooling and condensing excessive water vapor.